The present invention relates to a cellular-phone system applicable to a mobile communication system such as a radiotelephone system.
Heretofore, there has been developed a communication system called a cellular-phone system for making radio communication possible between a base station and a mobile station such as mobile telephone or portable telephone.
As shown in FIG. 1, a plurality of base stations BS01, BS02, BS03, . . . are located at a predetermined interval (e.g., at an interval of several kilometers) within a service area. When a mobile station MS existing within the service area communicates with a base station, the mobile station MS receives signals from base stations and judges on the basis of the received signals a most suitable base station (generally, base station located at the nearest position). Then, the mobile station MS supplies a network connection request signal to this base station to set a radio communication network between it and the most suitable base station, thereby making a communication with a called person via the base station connected between it and the communication network.
In the case of the above cellular-phone system, a base station side adjusts a power of a transmission signal in response to a communication state with the mobile station connected to the base station via a communication network. Specifically, the base station side detects a reception level of a signal transmitted from the mobile station. If the detected reception level is high, then the base station lowers the transmission power. If the detected reception level is low, then the base station raises the transmission power. As a consequence, the reception level on the mobile station side can be made relatively constant. Therefore, it can be expected that the communication state can be held constant regardless of the distance between the mobile station and the base station.
In actual practice, however, the reception signal at the mobile station is interfered with by a signal from another base station so that, even when the transmission power is controlled as described above, it is impossible to improve a transmission characteristic considerably. Specifically, if the reception level at the base station, for example, is high, then the transmission power is lowered. In the reception state at the base station, there is then the large possibility that a received signal will be interfered with by some waves on the basis of the state of the transmission line. When the transmission power is lowered, a reception rate of interference wave is increased and a reception state at the mobile station is degraded. There is then the large possibility that a signal from the base station will not be received accurately.
Conversely, if the reception level at the base station is low, then the transmission output is raised. However, if the transmission power of the base station is raised, then such raised transmission power becomes a disturbing wave in the adjacent area of another base station, which the becomes a large interference source for hindering the other base station from making a communication.
FIGS. 2A and 2B are diagrams showing the changes of channel interference ratio (hereinafter simply referred to as CIR) obtained when a transmission power is controlled in the base station. FIG. 2A shows measured results of the change of CIR obtained when the transmission power is not controlled in the base station. FIG. 2B shows measured results of the change of CIR obtained when the transmission power is not changed in response to the reception level.
When the transmission power is not controlled as shown in FIG. 2A, study of transmission power distribution on the right-hand side of FIG. 2A reveals that the transmission power is constant, whereas the channel interference ratio (CIR) is distributed in a wide range. Accordingly, if the transmission power is controlled, there are then many mobile stations which cannot receive a transmission signal due to a large channel interference ratio.
When the transmission power is changed in response to the reception level as shown in FIG. 2B, the transmission power on the right-hand side is largely distributed before and after the reference level and a distribution range of the channel interference ratio (CIR) shown on the left-hand side is narrowed to some extent. Therefore, study of FIG. 2B shows that a reception state could be improved to a certain extent. However, although the transmission power is controlled as described above, the distribution range of the channel interference ratio (CIR) is still large so that there are many mobile stations which cannot receive a transmission signal satisfactorily.
To solve the aforesaid problems, it is proposed to prevent an interference by extending a length in which base stations using the same frequency are located, i.e., enlarging a so-called frequency re-use size. Specifically, when the base stations are located in the state shown in FIG. 1, base stations BS01, BS11, BS21, BS31 and BS41 (base stations shown by open circles in FIG. 1) use the same frequency band (first frequency band) for communication, base stations BS02, BS12, BS22, BS32 and BS42 (base stations shown by open triangles in FIG. 1) use the same frequency band (second frequency band) for communication, and base stations BS03, BS13, BS23, BS33 and BS43 (base stations shown by open squares in FIG. 1) use the same frequency band (third frequency band) for communication, i.e., there are prepared base stations of frequency bands which are used in communications of three kinds in total. In this case, three-frequency band base stations using the first, second and third frequency bands constitute one re-use size R.
When the mobile station MS receives a channel of the first frequency band, there is the large possibility that the mobile station MS will be able to communicate with the nearest base station BS01 most satisfactorily. However, in this case, the mobile station MS receives signals from other base stations BS11, BS21, BS31, BS41 each using the same frequency band for communication. As a result, the signals from these base stations BS11, BS21, BS31, BS41 become interference waves.
In order to lower the level of the interference wave, it is proposed to increase the scale of the re-use size R by increasing the kinds of the frequency bands used and by increasing the number of base stations composing one re-use size R (the number of base stations using different frequency bands). If the re-use size R is increased in scale as described above, then this cellular-phone system uses many more frequencies. As a consequence, the efficiency at which the cellular-phone system uses the frequencies is lowered.
Since the dynamic range of the reception level at the base station reaches several 10s of decibels, if the transmission power from the base station is controlled by the level proportional to such dynamic range, then reception waves with considerably different level ratios exist in the carriers. There is then the disadvantage that an adjacent channel interference ratio on the reception side is increased.